This invention relates to production from petroleum wells, and in particular, the delivery of chemicals downhole. The invention will find the greatest use in wells which produce gas. Thus, the background description and the description of the preferred embodiments of the invention will focus on gas wells.
Typically, as gas is produced from a well the production decreases as the pressure in the formation decreases with the quantity of gas produced from the well. Usually, a well produces not only gas but also liquids, such as water and petroleum condensates with some asphaltines. As the pressure in the formation decreases the volume and velocity of the gas flow slows. Initially, the flow rate and velocity may be sufficient to remove the liquids with the gas. At some point in time the flow rate of gas will be insufficient to carry liquids out of the well, e.g., carry water and petroleum condensates, to the surface. As a result, the liquid loading in the well will increase, and liquid will collect in the bottom of the borehole. This liquid will exert back pressure on the formation which will further decrease the production rate from the formation, and in some cases cause production to cease altogether. When production by natural reservoir pressure becomes uneconomical, artificial lift techniques can be utilized to increase well production. A number of artificial lift systems are known in the industry, including sucker rod pumps, gas lift techniques and plunger lift techniques. Also, it has been known to utilize a combination of plunger and gas lifting techniques within a well as discussed in U.S. Pat. No. 3,090,316 entitled “Gas Lifting System.” Depending upon well conditions, economics and many other factors, a suitable lift technique is selected. Each technique has certain advantages and disadvantages.
Conventional plunger lift systems, which are also known as free piston systems, utilize a plunger (piston) that is dropped into the well by closing the valve on the wellhead and stopping the upward flow of gas in the well. The plunger is “free” because it is not attached to a sucker rod, cable or other mechanism to pull the plunger to the surface. The plunger falls to the bottom of the tubing and onto a bumper or stop at the bottom of the tubing. Liquid in the tubing will flow around the plunger as it falls in the tubing. After pressure in the well has built up to a predetermined load, the valve at the wellhead is opened and the gas pushes the piston upward to the surface, thereby pushing the liquid on top of the plunger to the surface. This sequence can be repeated by closing the wellhead off and allowing the plunger to fall again to the bottom of the well. Another technique is the use of a bypass plunger which is designed so as not to require the well to be shut in. U.S. Pat. No. 6,209,637 entitled “Plunger Lift with Multi Piston and Method” relates to this technique.
Frequently, a well that is utilizing plunger lift is an older well. As a result, the bottom of the well is subject to corrosion, scale, paraffin deposits, deposits of petroleum distillates and other undesirable deposits. Frequently, treatment chemicals are deposited downhole. These treatment chemicals can include such things as soap, acid, corrosion inhibitors, solvents for paraffin and petroleum distillates, stabilizers and other known treatment chemicals. A number of techniques have been employed to deliver treatment chemicals downhole. These techniques have many drawbacks, especially when they are employed with plunger lift production methods.
One method of treatment is to continuously pump a small amount of treatment chemical into the well during production. The treatment chemical falls to the bottom of the well, where it mixes with other fluids and is drawn up with the liquid lifted by the plunger. This continuous treatment approach usually requires a conduit to deliver the chemical to the bottom of the well and uses an unnecessarily large amount of chemicals. Another method is to use a batch treatment that involves pumping liquid treatment chemicals down the borehole relying on a dead space below the perforations to retain residual chemical for a period of time. The method is not reliable, and is unsuitable for use in some wells due to lack of perforations to retain the residual chemical, or because well production rates are so high as to quickly remove the chemicals. Thus, there has been a need for a method and apparatus to provide treatment chemicals down a wellbore which is efficient in the delivery of chemicals, minimizes waste of treatment chemicals, and minimizes disruption to production from the well. The current invention has the advantages of repeatedly delivering chemicals to the bottom of the oil or gas well to prevent scale deposits and/or corrosion. The current invention has the advantages of being economical, requiring little in the way of additional equipment and allowing continuous treatment of the wellbore. Thus, the system is much more efficient than existing delivery methods, in that it can concentrate chemical where it is often most needed which is at and below the stop (seating nipple), and is much less expensive to install than the continuous chemical injection method.
The drawings illustrate certain preferred embodiments of the invention and like elements have been provided with like reference numerals to corresponding items between various drawings.